The present invention relates to a multi-stage non-blocking optical switch matrix having failure protection. In particular, the optical switch includes failure protection in the form of a capacity to provide an alternative route for any connection, including rearrangeable protection to provide alternative routes for multiple connections, as required, and to permit replacement of any failed submatrix module without service interruption.
As optical telecommunication networks have evolved over the years and have become more complex, a need has arisen for a matrix switching system capable of optically coupling any one of a large number of fibres to another. Furthermore, it is desirable for the switching system to be xe2x80x9cnon-blockingxe2x80x9d, i.e. the switching of one input fibre to an output fibre should not interfere with the light transmission of any other input fibre to any other output fibre.
In an article entitled, xe2x80x9cA Study of Non-Blocking Switching Networksxe2x80x9d, published in the March 1953 issue of The Bell System Technical Journal, Charles Clos explained the characteristics and advantages of multi-stage switching arrays. Today, Clos multi-stage electrical signal routers are widely known in the industry, and many commercially available routers are based upon Clos"" initial design.
A Clos non-blocking optical switch matrix is a modularized non-blocking switch that can be configured from 1xc3x97n switches, or switches of other dimensions, in a multi-stage (3, 5, 7 . . . ) architecture including a centre stage between external input/output modules. Advantageously, this structure relies on smaller simpler switch elements, and requires fewer lens-to-fibre connections, reducing the costs of the switch matrix.
It is possible to provide an overconnected Clos network with one more center stage matrix than necessary for non-blocking functionality. Since the overconnected Clos network remains non-blocking with the complete failure of one center stage submatrix, replacement of a failed center stage matrix is always possible. Replacing a failed center stage matrix requires rerouting only those signals that pass through it. An overconnected Clos architecture embodies several forms of protection.
First, failures that occur in any single center stage submatrix can be accommodated by rerouting the signals that are affected, up to and including the loss of an entire submatrix.
Second, it is guaranteed that there are two possible routes from any input module to any output module. Most failures in an input/output stage that block a single route can thus be accommodated. With switched mirror crosspoint arrays, such failures would occur if a single mirror failed to actuate into either the reflective condition in which the signal route changes between rows or columns or into the xe2x80x9cpassthroughxe2x80x9d condition where the signal remains in the row or column in which it entered the switch. When a switch fails to actuate into the passthrough condition, it forces a particular path for the signal involved. The desired path can always be completed but there is a small probability that one other signal may also need to be rerouted through the center stages. This is so-called rearrangeable protection. If a switch fails in an intermediate condition it blocks all paths for the signal involved. This state requires external protection.
An overconnected optical Clos network with 2M center stages is therefore internally protected against total failure of any single center stage submatrix and most failures on a single path in the input/output stage submatrices (modules). However, protection is still needed against failures in the external input/output stages. Furthermore, protection implies the ability to repair a damaged part without interrupting the functioning of the switch. To replace any failed external submatrix module will require interrupting all paths connected to it. The replacement must be carried out without service interruption.
It has now been found that both internal and external protection from system failure in a multi-stage optical switch matrix can be secured by providing interconnected modules in a first external stage. The modules are interconnected by rerouting ports for redirecting signals from an input/output port on one external switch module to another external switch module. Further protection can be provided through a center stage having a number of center stage switch modules, fully connected to the external modules, equal to twice the number of input/output ports and rerouting ports on an external module.
With this structure, a failure at any point in the switch sub-matrices can be avoided, so that strictly non-blocking performance can be obtained. In the case of a failure, a signal can be rerouted without rearrangement of other signal paths, and faulty modules can be removed and replaced without affecting non-blocking switch performance. To replace an external module requires the provision of alternate paths to reroute a specific number of input/output signals.
Accordingly, the present invention provides a multi-stage optical switch matrix having failure protection for switching P optical signals to a plurality of locations in a non-blocking manner, which matrix comprises a first external stage, at least one center stage and a second external stage, the first external stage being optically coupled to the second external stage through the at least one center stage. The first external stage has R interconnected external modules, where R=P/M. Each external module comprises M external input/output ports, at least one protection port having a protection switch optically coupled thereto, a plurality of redundancy switches, each optically coupled to one of the input/output ports, and a plurality of second ports for coupling input signals from the external module to a center stage or from coupling output signals from the center stage to the external module, wherein each protection switch is coupled to at least one redundancy switch of another external module of the external stage. Preferably, the at least one center stage has 2 (M+Z) modules where Z is the number of protection ports per one external module. Preferably and typically, each first external stage is functionally identical with each second external stage.
In accordance with the invention, there is provided a module for use in a switch matrix for switching P optical signals to a plurality of locations in a non-blocking manner, the matrix having a first external stage with R external modules, a center stage and a second external stage with R external modules, the module comprising M external input/output ports, at least one protection port having a protection switch optically coupled thereto, a plurality of redundancy switches, each optically coupled to one of the input/output ports, and a plurality of second ports for coupling input signals from the external module to a center stage or from coupling output signals from the center stage to the external module, wherein each protection switch is coupled to at least one redundancy switch of another external module of the external stage.
It is an advantage of the present invention that the switch matrix permits switch module replacement without interfering with the non-blocking switching capability of the system.
It is a further advantage that the present invention provides an efficient and cost effective switch matrix structure. The switch of the invention requires an overall size of (P+MZ)xc3x97(P+MZ), rather than the minimum Pxc3x97P. An estimate of the cost by counting crosspoints, with (typically) Z=1, yields a cost increase roughly proportional to 1+(M/P)2, while the corresponding cost for one-to-one protection is greater by a factor of 2. Since M is always less than P, the structure in accordance with the present invention is always less expensive than a one-to-one protection.